A multitude of processes for the preparation and/or purification of lactide and polylactide (PLA) have been described in the literature to date. However, it has to be observed that, even if their scientific interest is undeniable, the very great majority of these processes remain laboratory processes which could never be operated industrially. This is because they have recourse either to highly specific (if not unique) equipment having no equivalent on the industrial scale, which renders their extrapolation and/or management very hazardous or complicated; or to a very low productive output and/or the substantial use of consumables which prevent any economically profitable operation of the process.
In point of fact, although resulting from a renewable starting material (not dependent on oil) and benefiting from a biodegradability which makes it possible to envisage it as one of the solutions to the increasing problem of waste, PLA will find its welcome only in the context of a cost price comparable to those currently available for polymers of petrochemical origin of the commodity products sector.
Nevertheless, two processes resulting from the state of the art might meet these requirements.
The first is disclosed in U.S. Pat. No. 5,274,073 of Gruber et al.
Gruber et al. envisage an integrated process for the synthesis of PLA starting from a solution (more or less pure) of lactic acid and/or of one of its esters comprising:    1. in one or two stages, evaporation of the free water and of a portion of the bonded water, so as to produce an oligomer with a molecular mass of between 100 and 5000 amu;    2. mixing the depolymerization catalyst with the oligomer, followed by thermal cracking of the mixture with production of lactide in the vapor form;    3. selective condensation of the vapors, followed by fractional distillation, making it possible to recover a purified lactide; and    4. polymerization of the purified lactide by ring opening to produce PLA.
The second is disclosed in U.S. Pat. No. 5,521,278 of O'Brien et al.
O'Brien et al. envisage an integrated process for the synthesis of the purified lactide for PLA starting from an aqueous lactic acid solution comprising at least 50% by weight of lactic acid comprising:    1. evaporation of the free water and of a small portion of the bonded water, so as to produce an oligomer comprising a number of monomer units (n) of between 2 and 8;    2. continuing the evaporation characterized by a greater diffusion surface area for the polymer and making it possible to obtain an oligomer comprising a number of monomer units (n) of between 8 and 25, stages 1 and 2 being carried out in equipment having a structure characterized by a low iron content;    3. mixing the depolymerization catalyst, devoid of alkali metals, with the oligomer, followed by thermal cracking of the mixture at a temperature below 240° C. with production (a) of a vapor phase comprising lactic acid, water, lactide and entrained heavy oligomers and (b) of a liquid phase comprising the heavy oligomers;    4. extraction of the fraction in the vapor form (a), so that its residence time in the cracking region is less than 15 seconds;    5. selective condensation of the vapors, followed by fractional distillation, making it possible to recover, by an intermediate extraction, a prepurified lactide in the liquid form; and    6. melt crystallization of the prepurified lactide, so as to result in a purified lactide fraction characterized by a residual acidity of less than 6 meq/kg.
Although these two processes appear advantageous, they have a number of shortcomings which might render problematic their chances of being used in an economic and profitable way for the production of a PLA of the quality for the commodity products sector.
If the teachings of Gruber et al. are considered, it is noticed that, by this process, the quality of the lactide obtained is not sufficient to make possible the synthesis of a polymer (PLA) with mechanical properties corresponding to those of the various applications selected. This is because it is well known to persons skilled in the art that the lowest possible contents of residual water and residual acidity are required in order to obtain polymers of high molecular mass (mechanical properties) with a high conversion (mechanical properties, stability, yield) and in a short reaction time (chemical and thermal stability; productive output).
In point of fact, by the purification technology selected, namely distillation, it is impossible to obtain, first, an optically pure product [the vapor pressure curves of the various stereoisomers (L-lactide or L-LD, D-lactide or D-LD, meso-lactide or meso-LD) being much too close, which proves to be essential for applications requiring a degree of crystallinity of the polymer] and, secondly, a chemically pure product as, by their own admission, they recognise that they cannot totally avoid the opening of the lactide ring in the distillation column and thus the contamination of the lactide in the system.
If the teachings of O'Brien are considered, it is noticed, following the introduction of an additional stage, namely the melt crystallization, that the optical and chemical quality of the lactide is achieved. However, the new process recommended consists of an extensive sequence of different technologies which, first, increases the complexity in the management of the process and, secondly, renders problematic its economic profitability, both with regard to capital costs and production costs. Furthermore, if all the stages of the process (evaporations; thermal cracking and distillation), with the exception of the melt crystallization, are examined, they are all characterized by high operating temperatures, which is in contradiction with the rules of the art generally recommended in the context of the synthesis of a heat-sensitive product, such as lactide.